1. Field of the Invention
The present invention relates to a method of manufacturing a cathode active material for use in a nonaqueous electrolyte secondary battery and to a nonaqueous electrolyte secondary battery.
2. Related Background Art
In recent years significant advances have improved the performance of electronic apparatuses and reduced the size of each apparatus to realize portability. Thus, there arises a need for high energy density batteries adaptable to the foregoing electronic apparatuses. Under the above-mentioned circumstance, a lithium-ion secondary battery has become commercial which contains LiCoO.sub.2 serving as a cathode material and a carbon material serving as an anode material with which lithium can be doped and removed. The lithium-ion secondary batteries have been widely used in a variety of portable electronic apparatuses, such as camcorders, portable telephones and notebook computers.
At the present time, improvement in LiCoO.sub.2 has been attempted to furthermore improve the characteristics of the lithium-ion secondary battery.
For example, a contrivance has been disclosed in Japanese Patent Laid-Open No. 4-253162 in order to realize an excellent cycle characteristic enabling thorough charge and discharge. The contrivance has a structure that a material obtained by substituting at least one element selected from Pb, Bi and B substituted for a portion of LiCoO.sub.2 is employed as a cathode active material. According to the above-mentioned disclosure, lithium carbonate, cobalt carbonate and lead dioxide or bismuth oxide or boron oxide is crushed in a ball mill so as to be mixed, and then the mixture is baked in air. Thus, a required cathode active material is obtained. In Japanese Patent Publication No. 4-24831, Japanese Patent Laid-Open No. 7-176302 and Japanese Patent Laid-Open No. 7-176303, use of a composite oxide expressed by A.sub.x M.sub.y N.sub.z O.sub.2 (where A is at least one type selected from alkali metal elements, M is transition metal, N is at least one type selected from Al, In and Sn, and x, y and z satisfy 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq.1.00 and 0.001.ltoreq.z.ltoreq.0.10) is disclosed. As embodiments of the foregoing disclosure, Li.sub.1.03 Co.sub.0.95 Sn.sub.0.042 O.sub.2, Li.sub.1.01 Co.sub.0.95 In.sub.0.04 O.sub.2 and Li.sub.1.02 Co.sub.0.96 In.sub.0.04 O.sub.2 are disclosed. In the above-mentioned disclosure, lithium carbonate, cobalt oxide, stanic oxide or indium oxide or aluminum oxide are mixed followed by baking the mixture in air. Thus, a required cathode active material is obtained.
However, the lithium-ion secondary batteries are widely employed in electronic apparatuses arranged to be operated in various environmental temperature conditions from low temperatures to high temperatures as well as those arranged to be operated at room temperatures. In particular, notebook personal computers each having a high-speed central processing unit encounters rise in the internal temperature thereof. Since the mounted lithium-ion secondary battery is used for a long time at high environmental temperatures, satisfactory characteristics at high environmental temperatures are required for the lithium-ion secondary battery.
However, a fact has been found that the conventional lithium-ion secondary battery including the lithium cobalt oxide as the cathode active material thereof is capable of exhibiting an excellent cycle characteristic if it is thoroughly charged and discharged. Moreover, the capacity of the lithium-ion secondary battery can easily be reduced if it is used or reserved at high environmental temperatures.
Moreover, the lithium cobalt oxide including various elements as described above encounters a difficulty that another element cannot generally easily be substituted for cobalt to form a solid solution while a layered structure is maintained. If start materials and a state of mixture of the start materials are unsuitable, the performance of the active material deteriorates excessively.